The present invention relates to the imaging of a plate or board having a coating imageable by ultraviolet or visible radiation and more specifically to actinic imaging with simultaneous heating to improve the imaging process. The invention is particularly directed to the imaging of lithographic printing plates.
One type of imageable lithographic printing plate or other imageable plate is a negative-working, actinic plate which has a resin coating normally soluble in a developer and which is rendered insoluble when exposed to radiation, usually in the ultraviolet range. The plate is imaged by exposing the coating to the radiation in those areas corresponding to the image to be printed with those areas becoming insoluble and ink receptive. Another type of imageable lithographic printing plate is a positive-working plate which is also actinically imaged. This type of plate has a resin coating normally insoluble in a developer which is rendered soluble when exposed to the radiation.
Actinic imaging, whether it be by ultraviolet radiation or by visible radiation, can be accomplished by one of two techniques. One technique is to expose the printing plate through a film negative. The other approach is to serially scan the plate with small image spots or areas. This latter approach can be accomplished by digital laser imaging or by a method referred to as digital screen imaging which will be explained later. Although digital screen imaging is not digital imaging in the strictest sense of that term as will be explained, the term xe2x80x9cdigitalxe2x80x9d will be used herein to encompass both the digital laser imaging and the digital screen imaging which both involve imaging by serially scanning the plate with small areas or spots of the imaging radiation.
An object of the present invention is to provide an improved actinic imaging method for either negative-working or positive-working plates or boards such as lithographic printing plates or printed circuit boards. The invention involves the imaging of the plate to cause a reaction in the coating in the exposed areas by actinic imaging using either ultraviolet or visible radiation in combination with infrared radiation. The reaction may either solubilize or insolubilize the coating depending on whether the plate is positive- or negative-working. The infrared radiation operates to increase the localized temperature of the coating to a level at which the rate of the actinic imaging reaction is increased. Either the ultraviolet/visible radiation or the infrared radiation can be image modulated. The relative areas covered by the ultraviolet/visible radiation and by the infrared radiation can be varied such that the areas are superimposed or that the area of the ultraviolet/visible radiation closely trails the infrared area.
The invention relates to the imaging of plates or boards with imageable coatings such as lithographic printing plates and circuit boards. The invention will be described with particular reference to lithographic printing plates but it is to be understood that the description also applies to such other products. Lithographic printing plates with coatings which are imageable by ultraviolet or visible radiation wherein the radiation causes an insolubilizing reaction in the coating for negative-working plates and a solubilizing reaction for positive-working plates are well known. Typically, the insolubilizing reaction for negative-working plates is a crosslinking or photopolymerization reaction but other chemical changes may also insolubilize the coating and are within the scope of the invention. The insolubilized imaged areas become the ink-receptive plate areas and the non-imaged areas are removed with a typical developer solution. The coatings for such plates are well known in the art and are typically a diazo resin having reactive sites which are capable of being chemically altered by the radiation. A suitable diazo resin is the condensation product of 3-methoxy-4 diazo-diphenylamine and paraformaldehyde. Other suitable diazo compounds are described in a variety of prior patents including U.S. Pat. Nos. 5,998,095; 4,956,261; 3,406,159; 3,277,074; 3,311,605; 3,163,633 and 3,050,502. For positive-working plates, the coatings are normally insoluble in the developer solution and are solubilized in the areas exposed to the imaging radiation. Such coatings are well known and usually comprise a diazide, such as diazonaphthoquinone derivatives, mixed with or reacted with a phenolic resin. For a specific disclosure of diazonaphthoquinone derivatives, see U.S. Pat. No. 5,858,626. The mechanism of such coatings is described in the article, xe2x80x9cThe Molecular Mechanism of Novolak Resinsxe2x80x9d by Arnost Reiser appearing in the Journal of Imaging Science and Technology, Volume 42, No. 1, January/February 1998 on pages 15 to 22.
Lithographic printing plates may be imaged by digital laser imaging and by digital screen imaging. The invention will first be described with respect to digital laser imaging and the digital screen imaging will be described later. Also, the invention will be described with respect to negative-working plates with coatings which are insolubilized by the radiation but it is to be recognized that the invention applies equally to positive-working plates with coatings which are solubilized by the radiation. Furthermore, the invention is applicable to imaging radiation with a wavelength shorter than infrared. For convenience purposes, ultraviolet radiation or devices will be referred to but it is to be understood that the imaging radiation of the invention includes visible as well as ultraviolet radiation. For example, blue or violet diode lasers or a double frequency YAG laser at 532 nm can be used. Like most chemical reactions, the rate at which the solubility conversion takes place for the actinic digital imaging of a lithographic plate is dependent on the temperature. The rate of reaction is slower at ambient conditions than at elevated temperatures. By elevating the temperature at which the imaging is done, the speed of the imaging process can be increased. It is problematic to increase the temperature of the entire plate by elevating the temperature of the imaging platen or drum. The thermal expansion of the components of the imager and the dynamics of managing the heat flow to the plate in a timely and uniform manner make this approach impractical.
According to the present invention, the temperature of the resin coating is increased to enhance the reaction but it is only locally and momentarily heated and heated generally simultaneously with the imaging. The invention involves spot heating the coating with an infrared laser and subjecting the coating while it is heated to the ultraviolet radiation to react the coating at the elevated temperature. The coating is not reacted or otherwise imaged by the infrared radiation but the infrared radiation interacts with the coating and/or substrate merely to heat the coating. Either the ultraviolet radiation or the infrared radiation can be image modulated as discussed below. If desired, an infrared absorbing dye may be incorporated into the coating to facilitate the infrared absorption and heating. These dyes are well known in the art and include materials such as squarylium, croconate, cyanine, phthalocyanine, merocyanine, chalcogenopyryloarylidene, orzindollizine, puinoid, indolizine, pyrylium, thizine, azulenium and xanthene dyes. The infrared heating of the coating can also be effected by using a substrate having infrared absorption characteristics. For example, an anodized aluminum substrate which has been rotary brush grained with calcined alumina so as to embed the graining particles to cover a portion, perhaps 10%, of the surface of the substrate will effect the rate of heat absorption by the substrate and the conduction of heat away from the spot by the aluminum. The power and intensity of the infrared laser as well as the dye in the coating and the nature of the substrate can be selected to instantaneously increase the coating temperature to enhance the coating reaction by increasing the reaction kinetics. The effect of the coating temperature on the reaction kinetics is progressive, i.e. the higher the temperature, the faster the reaction. Therefore, any temperature increase will have some effect but the preferred coating temperature range is 250 to 400xc2x0 F. With the higher coating temperature, much less power or time is required to trigger the reaction. Specifically, with respect to ultraviolet radiation, imaging can be accomplished with much less powerful gas lasers or with a diode or semiconductor ultraviolet laser in the power range of one watt or less. Therefore, the lasers are much less expensive, require less power input, do not require water cooling and are easier to modulate. The ultraviolet radiation is usually in the range of 340 to 390 nm.
In the practice of the invention, either the infrared radiation or the ultraviolet radiation may be image modulated. With modulated ultraviolet radiation, the unmodulated infrared radiation heats an area of the coating which is simultaneously imaged with superimposed modulated ultraviolet radiation. The spot of modulated ultraviolet radiation can be smaller than or the same size as the spot of infrared radiation. Alternately, the modulated spot of ultraviolet radiation can closely trail the unmodulated spot of infrared radiation with the criteria being that the coating is still hot at the time it is subjected to the ultraviolet radiation. With modulated infrared radiation, an area of the coating is exposed to a relatively low level of unmodulated ultraviolet radiation and that spot is simultaneously exposed to the superimposed modulated infrared radiation. The infrared spot is either the same size or smaller than the ultraviolet spot. The level of ultraviolet radiation is low enough that it will not significantly react the coating at ambient temperature during the short exposure time. It is only because of the simultaneous heating that the ultraviolet radiation is sufficient to react the coating in the heated image pattern formed by the infrared radiation.
In order to locally heat the coating and simultaneously image, the spots on the plate from the infrared laser and the digital imager must be located such that the imaging is effected while the coating is hot. This usually requires that the spots be superimposed although the ultraviolet spot may closely trail the infrared spot as noted earlier. Although the spots can be the same size, in order to facilitate superimposition and taking into consideration the ability to aim and focus the lasers, one spot is preferably larger than the other with the small spot being entirely within the bounds of the larger spot. The small spot is the modulated radiation whether that be ultraviolet or infrared. Because this ultraviolet imaging is a progressive process (rather than a threshold process) in which exposure of the coating to moderate levels of the ultraviolet radiation could produce some imaging, the ultraviolet spot is usually the modulated imaging radiation and is the smaller of the two spots with the ultraviolet spot being fully contained within the bounds of the infrared spot. Merely as an example, the large infrared spot may have a diameter of 50 microns while the small digital image spot has a diameter of 10 microns. If the infrared radiation is the modulated radiation, the infrared spot will be the smaller and the level of the ultraviolet radiation is kept quite low so that any coating reaction caused solely by the unmodulated ultraviolet radiation is minimal and insufficient to produce an effective image. Although this description of the invention only contemplates a single pair of infrared and ultraviolet lasers, there can simultaneously be multiple infrared and ultraviolet lasers.
Another method of imaging within the scope of the present invention is by a digital screen imaging system such as marketed by basysPrint Corp. This system uses a radiation source which is usually an ultraviolet light source that passes through a condenser system onto a digital screen. This screen has a grid with hundreds of thousands of dots that can each be individually electronically controlled. This produces a digital image on the screen that is projected onto the printing plate. It can be seen that it is the screen image that is digital whereas the image on the plate is a projected image from that digital screen. However, as previously stated, this imaging technique is encompassed with the scope of the digital imaging of this invention. Because of the size of the digital screen, only a segment of the plate is imaged in a single exposure. The segments range in size from 0.3 cm2 to 2.5 cm2 and can be projected at the rate of about 10 segments per second. The exposure head is moved or scanned over the plate to fit the segments together and produce the entire image on the plate. With this type of digital imager, one or more infrared lasers are positioned off to the side and aimed at an angle to focus on the area beneath the digital screen.
The imaging of the invention may be practiced on any conventional imaging equipment such as flat bed imagers, internal drum imagers and external drum imagers. In order to establish the principal and effectiveness of the present invention and the benefits achieved when imaging a coating at an elevated temperature, conventional plates with ultraviolet imageable coatings were heated to bring the coated plates up to a selected temperature. The heated plates were then imaged with ultraviolet light. The results were compared to unheated plates and to plates which had been heated but cooled prior to imaging.